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Craigieburn Rimu – How Old Are They?

Recently the Society have been wondering what age the 52 mature rimu trees are at Craigieburn. It’s a very common question asked by visitors to the site and one that is difficult to answer with any real certainty. Recently as part of the project the trees were mapped with GPS technology to enable mapping of the physical changes to the vegetation brought about by colonial occupation. While undertaking the mapping project the girth of the rimu trees were measured. Using some comparable growth data from other bush areas the ages of the trees was estimated at between 250 and 550 years old. However, the issue with this data is that it comes from studies undertaken outside of the Dunedin ecological district where climatic conditions are quite different from what we experience here in Dunedin.

The best way to age a tree is to count the growth rings, but in order to do this it has to be felled first so that the cut edge can be examined. The second method is to bore a hole with a core sampler and count the rings from the core sample. This has obvious risks to the health of the tree through moisture and bacteria getting into the core sampling hole. Ten or twelve years ago a significant rimu tree was struck by lightning in the water catchment land below Flagstaff. The rural fire officer at the time made the difficult decision to cut the tree down in order to extinguish the burning upper canopy and the tree lay undisturbed in the bush ever since that fateful lightning strike. The Society did not want to harm any trees in trying find a better way to age the rimu so it approached the City Council to see whether a log could be cut from the felled tree in the catchment land. They agreed with conditions that the information from the analysis would be provided to other organisations who were interested in similar projects. With the help of Nick and Don from Asplundh a ring from the existing felled rimu was eventually procured. Sadly, the burnt rimu had suffered extensive fracture damage and was quite rotten due to moisture and huhu grubs. With the help of Taskforce Green the ring was carried down through the bush to be slowly cured and sanded so that the growth rings can be counted.

This information will help the Society and other researchers in being able to understand the growth of rimu in the region and the results should be ready for publication early in the new year. The Society will have the ring preserved so that it can be used for environmental education, further study and public display.

Thanks Frank, hope we can provide some meaningful information on the age of trees and their growth in the Dunedin district. It would you be very interesting to demonstrate the digital probing method at Craigieburn after the growth rings are counted to compare the methods. The sanding will take a while, but hopefully it can give us an insight.